A small-sized and thin-type image pickup apparatus has been mounted on a mobile terminal representing a small-sized and thin-type electronic instrument such as a cell-phone and a PDA (Personal Digital Assistant), which has made it possible to transmit not only voice information but also image information to the remote place one another.
As image sensors used for these image pickup apparatuses, there are used solid-state imaging devices including an image sensor of a CCD (Charge Coupled Device) type and an image sensor of a CMOS (Complementary Metal-Oxide Semiconductor) type. As a lens for forming a subject image on the image sensor, lenses which are made of resins on a mass production basis at a low price come to be used for achieving a reduced cost.
As an image pickup lens used for such the image pickup apparatus (which is called also as a camera module, hereafter) that is built in a mobile terminal, an optical system of the type of three-element structure composed of three plastic lenses and an optical system of three-element structure composed of one glass lens and two plastic lenses are widely known. However, there are technical limitations for achieving a good balance between further downsizing of these optical systems and mass productivity required for mobile terminals.
To solve the aforesaid problems, there has been proposed a method for mass production of camera modules (see Patent Document 1) wherein a large amount of lens elements are formed simultaneously on a wafer with a dimension of several inches in a parallel flat plate shape, through a replica method, then, the wafer and a sensor wafer are combined and are cut. Lenses manufactured through this manufacturing method are sometimes called wafer scale lenses, and the camera modules manufactured through this manufacturing method are sometimes called wafer scale camera modules. Patent Document 1 discloses an image pickup lens that makes it possible to correct aberrations by simultaneously forming a diffractive surface and a refractive surface on a lens substrate.
However, it is not easy to simultaneously form a diffractive surface and a refractive surface on a lens substrate, and a thickness at the center on the refractive surface becomes extremely thin in the lens system in which an optical total length (a distance from an incident surface arranged at the closest position to the object side in the lens system, to an imaging plane of CCD) is small. Further, when the diffractive surface is employed, a diffraction efficiency for a wavelength other than a design wavelength is lowered, and an angle of light entering the diffractive surface is greatly restricted due to bad angle characteristics of an incident light entering the diffractive surface, which cause a problem that a wide angle of view is hardly secured.
Patent Document 1: Unexamined Japanese Patent Application Publication No. 2006-323365